1. Field of the Invention
The present invention relates to a recording/reproducing apparatus for a magneto-optical recording medium for recording or reproducing an information by using a laser beam, particularly to a magneto-optical recording/reproducing apparatus having an adjusting function for optimizing a reproducing or recording laser power in a method of recording/reproducing a minute mark of an optical detection limit or less by moving a domain wall of a recording magnetic domain known as domain wall displacement detection (DWDD).
2. Related Background Art
A magneto-optical disk drive is mainly used as a peripheral unit of a personal computer or the like, which has been developed as a large-capacity data storage unit because the medium can easily be carried and in which data can be repeatedly recorded or reproduced. Particularly, the drive has the superiority in data accessibility which cannot be realized by a tape-shaped recording medium, for example, it is possible to intermittently record continuous data in the drive or it is possible to reproduce an optional part from continuously long data at a high speed at random. Moreover, nowadays, a magneto-optical drive is further continuously improved in density and transfer rate and therefore, a purpose for a digital recording/reproducing apparatus for fine image information is opened as the main axis of multimedia unit.
Thus, in the case of a magneto-optical disk drive achieving a high speed and high density, it is important to adjust or optimize a laser power when recording or reproducing data. The optimum laser power is influenced by the track width, pitch, or groove shape of a disk (medium), or material sensitivity or uniformity in a radial direction of a recording magnetic film. In general, this difference depending on products is specific to a disk. When a disk differs, the optimum laser power differs. Moreover, even in the case of the same disk, aging occurs due to the warpage of the disk, contamination of the surface of the disk, or increase of use frequency or storage state of the disk. Therefore, because effective values of a laser power are changed, the optimum value of the laser power fluctuates. Moreover, in the recording/reproducing process of the magneto-optical disk, temperature is an important parameter and an optimum recording power or reproducing power is changed due to the temperature of the disk.
A laser power has been optimized so far by performing test-writing (trial recording/reproducing) in a test region formed at a predetermined position of a medium. For example, tests are repeated by preparing the data having a predetermined signal pattern and repeatedly recording and reproducing the data a plurality of times while changing laser powers. A laser power is optimized by considering the signal quality at each laser power in accordance with the jitter or error rate of obtained reproducing signals. The above optimization is executed whenever an apparatus is started or disks are changed, or every predetermined cycle.
Moreover, a method is proposed which optimizes a laser power in accordance with a measured temperature change by providing a temperature measuring function in a unit and thereby monitoring the temperature in an apparatus or surface temperature of a medium.
In recent years, various techniques are proposed for optimization of a laser power. Japanese Patent Application Laid-Open No. 8-221760 discloses a technique of obtaining an optimum reproducing laser power by slowly raising a reproducing laser power from a low power before starting a reproducing operation and thereby detecting a point at which a reproducing signal amplitude is saturated, and correcting the laser power by a predetermined value. Moreover, Japanese Patent Application Laid-Open No. 9-282729 discloses a technique of obtaining an optimum reproducing laser power by slowly raising a reproducing laser power from a low power before starting a reproducing operation and thereby detecting a point at which a reproducing signal amplitude rapidly rises, and correcting the laser power by a predetermined value.
These improved techniques make it possible to compensate a medium temperature change or the like caused by a temporal shift under actual reproduction for a prior art of obtaining an optimum reproducing power in a predetermined test region and thereby perform more optimum power control.
As the density of a recording medium and the transfer rate of data are improved, the above magneto-optical disk is decreased in diameter and it is studied to apply the disk to a compact mobile unit such as a disk camera for handling a large-capacity very-fine image information or disk camcorder. To such a unit for mobile purpose, in order to meet the requirement for portability in the outdoors, operability, size reduction, weight reduction, or the like, ultimate packing design is applied. Moreover, the airtightness in an apparatus is improved and the inside of a unit in use is brought into a considerably high temperature environment. Furthermore, the unit is exposed to a sudden environmental change such as indoor and outdoor traffic, temperature change, and humidity change.
Therefore, the magneto-optical disk drive is required to have an apparatus reliability to an environment change which is far higher than those for conventional computer peripheral units. Particularly, in the case of conventional computer peripheral units, a laser power can be optimized as needed by performing test-writing at a proper cycle. However, because the trends of magneto-optical disk drive markets are changed, a predetermined processing must be performed in real time when handing image information captured at a constant high-speed data rate. Therefore, frequent occurrence of adjustment time of test-writing causes critical traffic to increase in system processing.
This problem cannot be easily solved even by using the techniques disclosed in Japanese Patent Application Laid-Open Nos. 8-221760 and 9-282729 mentioned above. This is because each of the above techniques attains optimization of a laser power by monitoring the saturation or sudden rise of a signal reproducing amplitude value and providing a constant value for the amplitude value. However, the conventional power adjusting method of applying a uniform correction in accordance with a change in reproducing signal amplitudes cannot follow various fluctuation factors and therefore, it is impossible to accurately achieve optimization of a laser power. Specifically, static fluctuation factors include a difference in characteristics of individual disks (dispersion), and an irregularity in sensitivity characteristic (ununiformity) may occur at a specific portion of a disk. Moreover, dynamic fluctuation factors may include an environmental temperature change or a temporary warpage of a disk due to a sudden change of such temperature/humidity. Furthermore, a reflecting surface of a disk may be locally contaminated. Furthermore, the durability of a disk may change with the elapse of time. When any one of the above fluctuations occurs, optimum laser powers are not always equally determined even if change points of reproducing signal amplitudes are the same.
Therefore, the conventional techniques of correcting a laser power by a predetermined constant value in accordance with a change point of reproducing signal amplitudes has a problem that an adjustment accuracy becomes insufficient and therefore, a laser power cannot be optimized.